1. Field of the Invention
The invention relates in general to an apparatus and a method of fabricating semiconductor integrated circuits (ICs), and more particularly to an apparatus and a method of fabricating a polysilicon layer.
2. Description of the Related Art
Polysilicon is a pure material composed of single crystals of silicon grains that have various crystal orientations. In the other words, polysilicon is a pure material between crystal silicon and amorphous silicon. Grains in the polysilicon are separated from each other by a "grain boundary", which is a two dimensional defect. Since the grain boundary comprises many line defects and point defects, impurities permeate faster through the grain boundary than through the inner parts of the grains.
In modern very large scale integration (VLSI) processes, a polysilicon layer formed from heated and decomposed silane is deposited by low pressure chemical vapor deposition (LPCVD). If a reaction temperature is lower than 575.degree. C., the deposited silicon is amorphous. When the reaction temperature is between 575.degree. C. and 650.degree. C., polysilicon is formed. The higher the reaction temperature, the more manifest the crystallinity of the silicon. Basically, if deposition temperature of polysilicon is too high, the polysilicon forms toward homogeneous nucleation, which makes the polysilicon even less uniform.
In addition, it takes a longer time to deposit a polysilicon layer with a thickness of 1000 .ANG.. The longer time causes an annealing effect during the deposition and causes inclusions of particles in the deposited polysilicon layer as well as defects from homogeneous nucleation.
A conventional method of forming a thick polysilicon layer is provided to remove the particles and the defects described above. The polysilicon layer is formed using many steps of repeated deposition. A wafer is drawn from a chamber and cleaned between each deposition step. After the wafer is cleaned, it is put into the chamber, where continuous deposition is made on the wafer. An oxide film forms between the polysilicon layers while the wafer is placed in and taken out of the chamber. This oxide film can improve problems caused by particles and defects between polysilicon. However, repeatedly moving the wafer wastes much time and the additional step of cleaning makes the whole process complicated.